1. (a)When power-driven vessels are in sight of one another, a vessel manoeuvring as authorized shall indicate it by whistle: one short blast — 'I am altering my course to starboard'; two short blasts — 'I am altering my course to port'; three short blasts — 'I am operating astern propulsion'.
2. (b)Any vessel may supplement whistle signals by light signals of corresponding meaning, repeated as appropriate.
3. (c)When overtaking in a narrow channel or fairway: two prolonged plus one short — 'I intend to overtake on your starboard side'; two prolonged plus two short — 'I intend to overtake on your port side'; the vessel to be overtaken signals agreement with one prolonged, one short, one prolonged, one short.
4. (d)When vessels in sight of one another are approaching and either fails to understand the other's intentions or doubts whether sufficient action is being taken, the vessel in doubt shall sound at least five short and rapid blasts, which may be supplemented by a light signal.
5. (e)A vessel nearing a bend or an area where other vessels may be obscured shall sound one prolonged blast, answered by any approaching vessel within hearing.
6. (f)Whistles fitted more than 100 metres apart shall not be sounded simultaneously for manoeuvring and warning signals.